Precision rules of hardened steel and a knifelike edge or of glass with a semicylindrically rounded edge of approximately 0.2 mm. radius are known. They are used as auxiliary means for comparison measurements in mechanical machine shops and in making tools. To test the straightness of an edge or the flatness of a surface, the precision rule is placed and/or moved on a workpiece, whereby the gap which is visible between the edge and the workpiece allows light to shine therethrough to result in the measurement of the straightness or the flatness.
Already at small temperature variations, in particular triggered by the heat of the hands during working, the measuring edge deforms and inexactnesses in measurements result. To eliminate this or to at least delay this, such apparatus are made of a profiled rod, for example a five-sided, prismatic rod, of a hardened steel. Due to the demand for a stable form, the precision rule must have a relatively large cross section, which makes it heavy and difficult to handle, which results during the checking of larger workpieces in a quick fatiguing by the user. Furthermore, the workpiece which is to be measured can deform due to the weight of the precision rule which is placed on the workpiece.
Already the thermal process for the hardening of the steel leads to stresses in the precision rule which, during a later contact and therefrom the resulting heating up in certain locations can lead to additional deformation of the measuring edge. Further, different stresses result during the use of the precision rule in that same is held on the one side only by the thumb and on the other side, however, by four fingers, which results in a different heat effect. In addition, since hardened steel is a poor heat conductor, a localized heat accumulation can occur whereat the fingers and thumb engage.
All of the aforementioned irregularly distributed stress zones have a negative effect on the relatively thin measuring edge which, during comparison measuring, is the only active one and there exists the possibility of an error. The measuring process must be interrupted.
In order to avoid these heat influences, it has been suggested to provide the handle portion on the area of the precision rule remote from the measuring edge, with plastic or wooden bars functioning as a handle surface. These bars delay the heat flow, however, do not prevent it. In wooden handles there exists the danger of absorbing moisture, which in turn leads to one-sided stress.
The purpose of the invention is to provide a precision rule which is light in weight, which is stable and which, during normal use, is not exposed to any thermal stresses.
This is inventively attained by a precision rule having a handle part and a measuring block which are connected by structure which permits minimal reciprocal movements and make a heat transfer more difficult.
The measuring edge remains practically uninfluenced by the stresses which occur in the handle part due to heat and due to the expansionlike separation of the handle part and the measuring part. In addition, there exists the possibility to manufacture the handle part of a lighter and possibly less expensive material than the measuring part. The measuring edge of the measuring part can be replaced and can preferably be manufactured of a more expensive material.